1. Field of the Invention
The present invention relates to a spool valve for use as a directional control valve in an actuator such as a power steering apparatus for use on a motor vehicle, a powered boat, or the like.
2. Description of the Prior Art
There are known spool valve assemblies for use as a directional control valve in an actuator such as a power steering apparatus for use on a motor vehicle, a powered boat, or the like.
One known spool valve is illustrated in FIG. 1 of the accompanying drawings. As shown in FIG. 1, the spool valve has a spool 101 axially movably disposed in a valve case 106. The spool 101 is axially guided by a guide sleeve 107 fitted over one end of the spool 101 and threaded in one end of the valve case 106. An annular groove 100 is defined jointly in an outer circumferential surface of the spool 101 and an inner circumferential surface of the guide sleeve 107. The annular groove 100 houses therein a pair of axially spaced spring seats 102, 103 that are axially slidable in the annular groove 100, and a centering spring 104 interposed between and acting on the spring seats 102, 103. The spring seats 102, 103 and the centering spring 104 serve as a neutral position holding mechanism, or a self-centering mechanism, for returning the spool 101 to a neutral position with respect to the valve case 106. The spool 101 is maintained in the neutral position by the neutral position holding mechanism when no external forces are applied to the spool 101. However, as the guide sleeve 107 is not positionally adjustable, the changeover timing or position of the spool 101, i.e., the timing or position for the spool 101 to switch from one position to another, suffers an error depending on the machining accuracy of the spool valve.
Another conventional spool valve is shown in FIG. 2 of the accompanying drawings. The spool valve shown in FIG. 2 is a modification of the spool valve shown in FIG. 1, i.e., has a spacer 108, Belleville springs 109, and a set screw 110 in addition to the components of the spool valve shown in FIG. 1. More specifically, the spacer 108, which is of an annular shape, is axially slidably fitted over the spool 101 and held axially against ends of the guide sleeve 107 and the spring seat 103. The Belleville springs 109 are disposed between the spacer 108 and an inner axial wall of the valve case 106. The guide sleeve 107 is fixed to the valve case 106 by the set screw 110. In the spool valve shown in FIG. 2, since the spool 101 is positionally adjustable by loosening and retightening the set screw 110, an error of the changeover timing of the spool 101 can be eliminated by adjusting the position of the spool 101. However, the guide sleeve 107 tends to be loosened and positionally displaced because it is always subject to forces from the Belleville springs 109. The conventional spool valve shown in FIG. 2 is disclosed in U.S. Pat. No. 4,777,980.